1. Field of the Invention
This invention relates to a tool for removing a portion of a buffer tube used in a monotube type of optical fiber cable in order to selectively access optical fibers found therein.
2. Description of the Prior Art
The use of optical fibers in telephonic and other communication is well-known in the prior art. In order to protect the delicate optical fibers and to facilitate the laying of optical fibers, it is well-known to dispose a number of fibers in a monotube type optical fiber cable.
In a known cable assembly, radial strength yarns are helically or contrahelically wrapped around a single centrally elongate buffer tube, or "monotube", and the resultant structure is encased within a common protective sheath of steel. Two elongate strength or support members are disposed on either side of the structure which is then further encased within a plastic material such as polyethylene, polyurethane, polyvinylchloride, etc. The interior of the cable, between the radial strength yarns, is impregnated with a resin or oil-like substance providing lubrication and water resistance. The monotube is filled with a moisture-resistant gel for providing a light buffer to external forces and further for improving the cable's resistance to water ingression.
While optical fibers have many advantages, such as high density and lack of electromagnetic interference, over traditional conducting cable in the communications field, they have a major disadvantage in that there is degradation in transmission efficiency after a fiber has been spliced or in any way joined with another fiber after the original manufacture. Such a splice may be inevitable in the event of fiber breakage, the rerouting of a communications line or the insertion of an intermediate device within the communications line. However, such a splice may only be required for a single or limited number of fibers within the monotube. Under such circumstances, it would be extremely time consuming and degrading to the communication efficiency to completely sever and subsequently splice all the optical fibers in a monotube in order to access a single optical fiber. It would therefore be advantageous to provide a device which would allow access to a limited number of fibers within a monotube design fiber optic cable without cutting or otherwise disturbing the remaining optical fibers.
AT&T Western Electric manufactures Unit Slitter Tool (961B or 961C) which consists of two rectangular sub-assemblies which are attached to one another by spring loaded shafts at each end. On the inside surfaces of these two sub-assemblies are plastic inserts with semi-circular grooves which hold the monotube in place for slitting when the sub-assemblies are closed. Extending inwardly into the semi-circular groove of each inside surface is a blade for cutting the monotube. A blade holder having two screws is utilized to hold the blade in place. Each blade is disposed so that it extends towards the center of the semi-circular groove and is parallel to the groove longitudinal axis.
In order to access an optical fiber, the monotube is placed between the two sub-assemblies so that it is disposed within one of the semi-circular grooves. The sub-assemblies are then compressed against each other thereby enclosing the monotube. As the monotube is enclosed, the two blades, one blade from each subassembly, are driven into the monotube. The user then draws the tool along to the longitudinal axis of the monotube thereby cutting the buffer tube in two and exposing the optical fibers. The two halves of the monotube are then removed leaving all fibers totally exposed and unprotected.
The AT&T Western Electric device exhibited several disadvantages. The monotube is cut by two blades each extending towards the center of the tube resulting in a more intrusive device than is sometimes necessary. Such intrusion may result in unwanted nicking or severing of an optical fiber, especially if the blades are dirty, dull, improperly adjusted or loosely attached. In addition, there is no way to monitor for such a malfunction since the blade operation is hidden from the operator's view. In order to properly adjust the blades, blade adjustment requires the loosening of four screws, the insertion of a calibration standard and the adjustment of two set screws for blade heights. This is a tedious process which may actually inhibit the operator from consistently calibrating the blade height which may increase the probability of fiber damage during the use of the tool.
Since the tool is very large and awkward in shape it is difficult to manipulate and is ineffective for use in situations where there are constraints upon the amount of cable access space and fiber access length. For example, due to the device's length, at least ten inches of monotube must be exposed in order to access six inches of fiber length. For shorter monotube access lengths, this tool is not usable. In addition, the tool does not allow the operator to easily access fibers without severing the rigid strength members of the monotube fiber optic cable. In many installations, it is desirable that the strength members are left intact to provide additional tensile strength in the closure. The AT&T tool is difficult to use when the strength members must be left intact.
Alcatel Cable Systems manufactures a fiber access tool for accessing optical fibers in buffer tubes of loose-tube design optical fiber cables, described in U.S. Pat. No. 4,972,581, which is sold under the designation `pocket shaver`. The Alcatel fiber access tool comprises three major parts--a body, a clamp and a blade. The body is roughly cubical in shape and has an open channel adapted to receive a portion of the cross section of a buffer tube such as those found in loose-tube optical fiber cables. The clamp is designed to be slidably received onto the body thereby closing the channel and engaging the loose-tube buffer tube between the body and the clamp. The clamp presents an edged surface in order to allow the clamp to be inserted between a buffer tube and a cable when only a little slack is present.
The blade of the Alcatel fiber access tool is adjustably attached to the body so that the edge extends into the channel. The edge of the blade is perpendicular to the axis of the channel and defines a chord across the circular cross-section of the channel. The blade can remove a section of the buffer tube as the tool is moved along the tube, thereby allowing access to the optical fibers therein.
The Alcatel `pocket shaver`, however, may be used only with those optical fiber cables having helically or contrahelically wrapped buffer tubes, such as those found in the loose-tube optical fiber cable or a tight buffered optical fiber cable. The `pocket shaver` fiber access tool cannot be utilized for accessing the optical fibers i monotube type optical fiber cable. Since the tool is very small and lightweight, it is ineffective for use in applications where the buffer tube to be cut is much larger and more rigid than loose-tube type buffer tubes, such as a monotube. In addition, the tool is not designed to allow the operator to access the fibers of a monotube fiber optic cable without severing the rigid strength members of the cable. In many installations, it is desirable that the strength members are left intact to provide additional tensile strength in the closure. Thus, the Alcatel `pocket shaver` is difficult to use for monotube fiber optic cables and cannot be used without severing at least one of the elongate strength members.
As can clearly be seen, providing a fiber access tool for use with monotube fiber optic cables is a difficult problem which has not been previously resolved and even with the current level of understanding of optical fiber cables, there has not previously been a practical optical fiber access tool which may be used on a monotube fiber optic cable although such a tool is desirable.